According to a text of that time,  “Nothing was so grandiose as the observations carried out on June 21st 1667, day of the solstice … Since this building [the Observatory] was destined for science, and mainly for astronomical observations, it had to be built on the meridian, and all its angles had to satisfy certain directions. The mathematicians thus came there on June 21st.” These mathematicians, members of the very new Academy of Sciences–Auzout, Frénicle, Picard, Buot and Richer­–“were from daybreak at the Uranoscope or at the Observatory, in order to engrave a meridian line on a stone plate that had been placed by M. Couplet.”

caption : Foundation of the Observatoire de Paris - copyright : Observatoire de Paris

To know more : The beginnings of the Royal Observatory